The present invention relates to power control systems which comprise a prime mover and a generator driven by the prime mover. Such a power system may be implemented in a vehicle, where an internal combustion engine provides mechanical power to propel the vehicle, and to drive engine accessories, such as generators, air conditioning units, compressors, cooling fans, and pumps, to name a few examples. In a generator set, an internal combustion engine drives a generator to convert the engine's mechanical power into electrical power. The present invention specifically focuses on a control device that controls the generator output power in order to avoid excessive load on the engine and the drive components. In particular, the control device is configured to ascertain a change in speed of the generator or the engine and to control the generator output power controller to avoid stalling the engine.
Electro-mechanical power conversion systems, such as those mentioned above, are normally comprised of an internal combustion engine and a generator. The engine supplies the generator with mechanical power where it is converted to electrical power. In a vehicle, for instance, the generator generates electrical power for the vehicle electrical system when the vehicle's engine is operating. In a generator set, the engine's mechanical power is converted to electrical power by the generator which is available via power output connectors. As electrical loads are added and removed from the generator, the engine experiences the corresponding variation in mechanical loads. In the case of the vehicle, during idle periods, such variations in mechanical loads on the engine cause the engine's rotational speed, commonly referred to as the RPM (revolution per minute) to vary accordingly. In the case of the generator set, similar changes in the RPM occur as electrical loads are connected and disconnected with the power output connectors.
The engine drives the generator, via a coupling, at a substantially equivalent speed. A typical coupling between an engine and a generator comprises a pair of pulleys and one or more belts that are used to impart the engine's speed onto the generator. Another type of coupling involves splined shafts where specially machined shafts of the engine and generator are mated together so that the engine's shaft directly drives the generator's shaft.
Engine stall is a common problem in electromechanical power conversion systems, such as those mentioned above. Engine stalls when there is excessive mechanical power demand on the engine. This demand can be due to electrical loads on the generator or any other mechanical power consuming device driven by the engine. A typical internal combustion engine power output is a function of the engine RPM. At steady state conditions, the engine can deliver a certain torque at the operating RPM. As the torque requirement exceeds this torque the engine RPM begins to decrease which may eventually cause the engine to stall. In addition, a rapid change in the power transfer, can damage the drive mechanism.
Furthermore, sudden power demand from the engine can be hindered by the existing mechanical loads on the engine. Such mechanical loads can be due to a generator and/or an accessory, such as an electrical motor. The mechanical load on the engine, due to the generator, can be excessive. Removal or attenuation of such load from the engine can supply the engine with the power it needs to meet the sudden power demand.
Although various systems have been proposed which touch upon some aspects of the above problems, they do not provide solutions to the existing limitations in power control systems. For example, in Blackburn, U.S. Pat. No. 6,801,020, the invention is directed to a method of limiting a rate of change of the output current supplied by a starter/alternator in the generator mode, thereby limiting the possibility that the rapidly increasing electrical load could either stall or cause another operational fault in the associated internal combustion engine. The present invention focuses on a change in the speed of the generator and controls its output according to the change. Accordingly, any change in speed, regardless of the source that caused such change, will be monitored and acted upon.
In Fenley, U.S. Pat. No. 5,570,001, the invention discloses an apparatus that includes an engine driving an alternator where the engine speed is automatically controlled by manipulating the throttle according to the charging current of the alternator. The apparatus is further capable of unloading the alternator from the engine when excessive electrical loads are engaged, in order to prevent the engine from stalling. However, since this apparatus operates on detecting the alternator current, it is prone to electrical noise. The system described herein monitors a change in speed which is a mechanical action rather than monitoring an electrical variable. Furthermore, electric current at a fixed voltage is related to electric power which in turn is related to the mechanical power produced by the engine. That is why Fenley teaches unloading the alternator at an excessive current. But the engine mechanical power varies also with combustion air density, fuel energy content, ambient temperature, wear condition of the engine, other parasitic loads driven by the engine, and various other conditions that Fenley needs to consider in calibrating engine overload power. The system described herein does not need to consider such influences to the available engine power because a sudden reduction of engine speed is presumed to be an overload on the engine power, no matter the source of that overload.
In DeBiasi et al., U.S. Pat. No. 5,481,176, the disclosure describes a charging system including an engine driving an alternator and a voltage regulator, where the voltage regulator voltage set-point is modified by an engine controller device according to (1) near-wide-open-throttle, (2) application of vehicle brakes, and (3) increased torque of the alternator, or any combination thereof. When condition (3) is met, the engine controller manipulates the engine idle speed to keep it relatively constant as the applied electrical loads cause the alternator's torque on the engine to increase. However, this charging system monitors an increase in alternator torque on the engine, whereas the present invention monitors a change in the rotational speed of the alternator. Additionally, the DeBiasi system responds to an increase in torque of the alternator on the engine, whereas the present invention responds to a change in torque on the engine regardless of the source, even torque changes from an accessory other than the alternator that is driven by the engine.
In a copending commonly assigned U.S. patent application Ser. No. 11/234,579, filed Sep. 23, 2005 and entitled “Power Control System and Method,” hereby incorporated by reference in its entirety, a power control system was disclosed where a control device ascertained a power level of the generator and varied an output power of the prime mover according to the power level. More specifically, the prime mover output power was manipulated so that the generator power level could be maintained within a specified range. The present invention complements this system by providing an additional protection to the engine from stalling during those periods when the change in engine rotational speed is so high that a faster response than manipulating the engine's output power is required.
Power conversion systems, such as those incorporated in a vehicle or a generator set, utilize a prime mover and a generator. The output power of the prime mover may be adjusted in accordance with the power level of the generator. Additional protection can be afforded such systems by monitoring the change in the rotational speed of the generator or the prime mover and manipulating the output power of the generator to avoid excessive loads on the engine. Where a situation arises that the prime mover output power adjustment does not adequately address the excessive load problem of such prime movers, the present invention provides an additional protection by adjusting the output power of the generator.